Apocrine Gland Adenocarcinoma in Dogs

Apocrine gland adenocarcinoma of the anal sac is a tumour accounting for 17% of perianal tumours in dogs. Anal sac adenocarcinoma is a disease of older dogs with the average age of onset being between 10 and 11 years. Benign tumours of the anal sac are rare and for this reason, rectal examination is recommended as part of the routine physical examination for every dog over 5 years of age.

A high metastatic rate has been reported, ranging between 46 and 96%, at diagnosis. The sublumbar lymph nodes are the most common site for metastasis. However, metastatic involvement can also be noted in the spleen, liver and lungs. Anal sac adenocarcinoma can, in 27% of cases, produce the PTH related peptide (PTHrp). PTHrp is similar in structure to PTH and, over production of PTHrp by the tumour can result in hypercalcemia. Hypercalcemia is the most common paraneoplastic syndrome associated with this tumour.

When a mass of the anal sac is discovered, it must be evaluated. Fine needle aspirate and cytology are the first steps. Cytology has an excellent positive predicative value but a poor negative predictive value. This fact means any negative cytology result must be followed with a biopsy. Staging includes three view chest radiographs and abdominal ultrasound.

Tumour size is prognostic. Tumours less than 5cm in size have a 60% 2 year survival. If the regional lymph nodes are involved at the time of diagnosis, the survival time, regardless of therapy is approximately 7months. Hypercalcemia tends to be associated with a poorer prognosis.

Anal sac adenocarcinoma was commonly thought to have a poor prognosis. However in truth, this tumour has a good prognosis and, while rarely cured, can be treated as a chronic disease and managed for excellent long term survival. Treatment for this tumour typically involves a combination of local and systemic control. Complete excision by surgery is the preferred method for local control. There are many reports of adding radiation therapy to follow surgery for an extended survival time. Chemotherapy is always recommended following local control.

There is no statistical difference between protocols including radiation therapy and those that do not. Radiation is often not included due to limited access or to concerns over morbidity.

1 Year Survival

2 Year Survival

Median Survival Time

Chemotherapy Alone

–

–

212 days

Surgery Alone

65%

29%

500 days

Radiation Alone

79%

38%

657 days

Combination Therapy

80%

56%

742-956 days

This disease may reoccur. Recurrent disease is addressed with local control and another round of chemotherapy. It is not unusual to go through two or three cycles of treatment. Survival times can approach 3 years suggesting this tumour is well suited to long-term survival and control. Recent developments with tyrosine kinase inhibitors have demonstrated a new avenue to gain even longer survival times. This tumour can express the Cox-2 receptor and inhibition with a Cox-2 inhibitor may also provide a survival benefit.

Dr. Kevin Finora is a Board Certified Oncologist and Small Animal Internist who is part of the Healthcare Team at the Central Toronto Veterinary Referral Clinic. He is available for referrals and consultations Monday to Thursday (including Monday and Tuesday evenings). Please contact him with any oncology questions or concerns.